1. Field of the Invention
This invention relates to irradiated polyethylene having superior film fabrication properties. More particularly, the present invention relates to low level irradiated, linear, low density, ethylene/alpha-olefin copolymers and to processes for the manufacture of film from such material.
2. Description of the Prior Art
Linear, low density, ethylene/alpha-olefin copolymers which are substantially free from long chain branching and have a narrow molecular weight distribution, hereinafter referred to as "LLDPE", exhibit a superior balance of mechanical properties when compared with other polyethylenes. Low density polyethylenes, both LLDPE and conventional, long chain branched (LCB), possess a superior balance of properties to high density polyethylenes in overall tensil strength, impact resistance, burst strength, tear srength, low temperature strength and flexibility, chemical resistance and dielectric strength. In addition, LLDPE is superior to LCB low density polyethylene in withstanding increased drawdown, which can result in unbalanced film properties and greater production expenses for LCB polyethylene. The charactertistics of LLDPE in providing for a superior balance of mechanical properties in combination with improved film fabrication rheology, distinguishes it as a versatile material having broad utility.
The unique attributes of LLDPE also, however, create difficulties in certain applications, such as film fabrication. For example, LLDPE has limitations in achieving commercially desirable film quality at high film fabrication rates without incurring difficulties, such as, film bubble instability during blown bubble expansion, or draw resonance during melt embossed slot cast or extrusion coating processes.
Solutions to such problems have included mechanical modification in the processes and apparatus used in forming blown film, as disclosed in U.S. Pat. No. 4,330,501 (Jones et al.), involving the modification of cooling fluid flow. Improved extrusion coating processing has been achieved by utilizing blends of LLDPE with LCB low density polyethylenes, as disclosed in U.S. Pat. No. 4,339,507 (Kurtz et al.).
Radiation treatment has been utilized extensively to modify polyolefin rheology. Much of the prior art pertains to radiation treatment of preformed film in order to improve certain mechanical properties. For example, British Pat. No. 2,019,412 (Clarke et al.) pertains to the irradiation of LLDPE film using dosages of between 2 to 80 Mrads, to provide increased elongation at break values. Low level radiation of between 0.05 to 0.25 Mrad, is disclosed in U.S. Pat. No. 3,563,870 (Tung et al.) to improve melt strength and melt extensibility in high density polyethylenes. Low level irradiation has also been disclosed to reduce the melt index and increase the intrinsic viscosity of low crystallinity, LCB low density polyethylenes as set forth in Canadian Pat. No. 788,440 (Sacks et al.). In an article entitled "Crosslinking Effects on Flow of Irradiated Polyolefins," by Markovic et al., in Modern Plastics, October 1979, at page 53, involving low level irradiation of polyolefins, including polypropylene, high density polyethylene and various LCB low density polyethylenes, it was noted that such irradiation has a greater effect on absolute viscosity at low shear rates than at high shear rates. Irradiation of polyethylenes in general, however, may often be undesirable in reducing surface properties, such as smoothness, required in various coating applications.